1. Field of the Invention
This invention relates to image display systems and, more particularly, to display systems for displaying a video signal intended to be recorded on a recording medium, or a video signal reproduced from the recording medium.
2. Description of the Related Art
In the following written specification, taking an example of the home video tape recorder (VTR), this kind of system will be described below.
In general, the home VTRs have been known to be of the so-called rotary two-head helical scanning type. FIG. 1 is a diagram illustrating the head arrangement of this type VTR. In FIG. 1, a magnetic tape 1 is wound around the outer periphery of a rotary drum 3 over an angular distance of 180.degree. or more by tape guides 2a and 2b.
Rotary heads HA and HB are mounted on the rotary drum 3 in phase difference of 180.degree. from each other, and have different azimuth angles from each other. As is well known, for 180.degree. rotation of the heads HA and HB, every one field of video signal is recorded or reproduced.
Since, in this kind of VTR, by the standard, the track length for one field of video signal is predetermined, leading to the necessity of taking the diameter of the rotary drum 3 at a certain value. From this reason, the drum diameter could not be reduced any more, hindering reduction of the size and weight of the VTR. Proposals, as described below, for a VTR capable of reducing the drum diameter have been made and put into practice. FIG. 2 is a diagram illustrating the head arrangement of the conventional VTR having the small-diameter drum. In the figure, Ha and Hb denote rotary heads of different azimuth angles from each other which rotate one revolution during the one-field period of video signal. The tape 1 is wound round the drum 4 to an angular range of more than 300.degree. by tape guides 5a and 5b. The rotary heads Ha and Hb each rotate 300.degree. when one field of video signal is recorded. In other words, the video signal for one field is caused to be recorded in a shorter period than that of the original one field of video signal.
Therefore, when the video signal to be recorded by this kind of VTR is assumed to be NTSC signal instead of the normal NTSC signal, that is, the signal whose vertical scanning frequency (fV) is 60 Hz and whose horizontal scanning frequency (fH) is 15.75 kHz, a one of 60 Hz in fV and (15.75.times.(6/5)).apprxeq.18.9 KHz in fH must be used.
In short, the video signal to be recorded by this kind of VTR must be what is obtained either by compressing the time axis of the normal television signal to 5/6 in each field, or from a specified camera therefor.
The specified camera has a picture frame (shown by dashed lines Y in FIG. 3) whose aspect ratio is 9:10. On scanning, that partial area which has an aspect ratio of 3:4 shown by solid lines X in FIG. 3 is taken as the effective picture frame to be produced within 5/6 of the field period, and to be recorded by the heads Ha and Hb.
The timing of this recording is explained by the timing charts of FIGS. 4(a) and 4(b). In the figures, in the period denoted by T, the video camera scans the picture frame Y of the aspect ratio 9:10 to obtain a video signal consisting of continued 315 horizontal scanning lines as shown in FIG. 4(a).
However, the time during the head Ha or Hb actually traces on the tape 1 accounts for 5/6 of the period in each field. Out of the 315 lines, therefore, 262.5 horizontal scanning lines are taken out as shown in FIG. 4(b) and recorded as the video signal of aspect ratio 3:4 on the magnetic tape. At this time, the heads Ha and Hb alternately trace on the magnetic tape. Hence it is possible for the VTR of such a head arrangement as shown in FIG. 2 to perform a similar recording to that in the VTR of the head arrangement shown in FIG. 1. Yet, because there is need to give a slight phase difference between the heads Ha and Hb, either the first field or the second of video signal must properly be delayed.
The use of such a head arrangement as shown in FIG. 2 in combination with such a method for recording the video signal as described above makes it possible to reduce the drum diameter to 3/5 of that of the VTR of FIG. 1.
By the way, in the VTR having such a head arrangement as shown in FIG. 2, when the recorded video signal is reproduced, absence of that reproduced video signal occurs for 1/6 of the period in every one revolution of the head. Also, since the horizontal scanning frequency fH is different from that of the normal television signal, it cannot be displayed on the television set or the like as it is.
So, an arrangement which enables the video signal to be reproduced by using a variable delay line is disclosed in Japanese Laid-Open Patent Application No. Sho 60-19372. In the VTR disclosed by that publication, the reproduced video signal obtained in a 5/6-field period out of the one-field period (having such a reproduced RF waveform as shown in FIG. 4(b)) is expanded in the time axis by using the variable delay line to get a continuous reproduced video signal.
Also, a so-called field memory may be used for similarly expanding the time axis to 6/5 with the video signal for one-field period, as disclosed in U.S. patent application Ser. No. 034,838 filed (Apr. 6, 1987), now U.S. Pat. No. 4,870,509.
However, in the case when such techniques as described above are used for displaying the reproduced video signal, either the variable delay line of 0-1/6 field periods or a memory capable of storing one field of video signal is necessary, and further a control circuit for these becomes necessary. Therefore, the reproducing function could not be cheaply added to the VTR having the drum of such a small diameter as shown in FIG. 2.
Also, because the reproduced video signal is produced once through the variable delay line or the field memory, deterioration of the signal arises thereby negatively influencing the image to be displayed.
Also, in the video recorder of the one-body type with camera, a smaller size and a lighter weight are demanded. But, the provision of the control circuit using such a field memory as described above increases the scale of the apparatus, and, in the recorder of the one-body type with the camera having the electronic view finder, incorporation of the control circuit therein is not conducive to minimization of its size.
Also, in the case when the VTR having such an image receiver as described above in the unified form is used as portable, it is desired to add even a capability of receiving and displaying the television signal being broadcast.